The present invention relates to a time base error compensation device of a video signal in a helical scan type magnetic recording and reproducing apparatus having a plurality of magnetic heads, and more particularly to a skew distortion elimination device suitable for an apparatus which records or reproduces video signal on or from a magnetic tape by switching a plurality of magnetic heads during one vertical scan period.
In a magnetic recording and reproducing apparatus such as a VTR or a video reproducing apparatus such as a video disk player, a time base error is caused in a reproduced video signal by a relative positional movement between a signal detection device such as a magnetic head or pickup head and a record medium such as a magnetic tape or disk. If the time base error is slow, the image on the playback screen jitters, and if a rapid change (skew) is included in the time base, the image on the playback screen is distorted. Accordingly, the time base error of the reproduced video signal involves an essential problem of degrading stability of the playback screen.
In order to compensate for the time base error, a time base correction device as shown in FIG. 2 has been used in the past (Japanese literature, VTR Technology, Chapter 6, Broadcasting Technology Vol. 5, Nippon Hoso Kyokai).
In FIG. 2, numeral 10 denotes an input terminal to which a video signal including a time base error is applied, numeral 20 denotes an output terminal from which a video signal compensated for the time base error is outputted, numeral 1 denotes an A/D converter for converting the input video signal to a digital signal, numeral 2 denotes a memory such as a RAM, and numeral 30 denotes a horizontal synchronization signal separation circuit. A horizontal synchronization signal including the time base error, extracted by the horizontal synchronization signal separation circuit 30 is supplied to a write clock generator 40 and a write address control circuit 50.
The write clock generator 40 generates a write clock which reflects the time base error in the input video signal applied to the input terminal 10, in synchronism with the horizontal synchronization signal. The write address control circuit 50 produces a write address signal in response to the write clock.
Accordingly, the video signal including the time base error, applied to the terminal 10 is sequentially converted to a digital signal by the A/D converter 1 in synchronism with the write clock from the write clock generator 40 and written into the memory 2 in accordance with the address signal from the write address control circuit 50.
On the other hand, a stable reference synchronization signal free from the time base error is applied to a terminal 80, and a read clock generator 70 generates a read clock in synchronism with the reference synchronization signal. A read address control circuit 60 produces an address signal in synchronism with the read clock.
Accordingly, the video signal data stored in the memory 2 is sequentially read for each horizontal scan period in accordance with the address from the read address control circuit 60, and the read data is sequentially converted to analog signal by a D/A converter 3 in synchronism with the read clock generated by the read clock generator 70. Accordingly, a stable video signal free from the time base error is outputted at the terminal 20.
As seen from the above description of operation, the performance of the time base error compensation device is influenced by the method for generating the write clock by the write clock generator 40, and it is an important factor of the device how to generate the write clock which exactly follows the time base error of the input video signal.
The write clock generator 40 is usually constructed by an automatic frequency control (AFC) circuit shown in FIG. 3, in which the horizontal synchronization signal from the horizontal synchronization signal separation circuit 30 is applied to one input terminal of a phase comparator 43 through a terminal 41. Numeral 45 denotes a voltage-controlled oscillator having a center frequency which is substantially equal to the frequency of the read clock from the circuit 70 of FIG. 2. The output of the voltage-controlled oscillator 45 is frequency-divided by a frequency divider 46 which produces a signal having the same frequency as the horizontal scan frequency of the input video signal. The horizontal synchronization signal applied to the terminal 41 and the output signal from the frequency divider 46 are phase-compared by the phase comparator 43 which produces an error voltage representing a phase difference therebetween. It is supplied to the voltage-controlled oscillator 45 through a phase compensation circuit 44 as a control voltage.
The above circuits constitute the AFC circuit. Through a negative feedback control thereof, the voltage-controlled oscillator 45 produces the output signal which follows the time base error of the horizontal synchronization signal of the input video signal and the output signal is outputted from a terminal 42 as the write clock.
The prior art write clock generator has thus been described. Because of the negative feedback control, the AFC circuit of the prior art system does not correctly follow the time base error when the frequency of the time base error is high or the time base error occurs rapidly like a skew. If the response speed of the AFC circuit is increased to enhance the compensation capability, the AFC circuit is sensitive to a noise included in the input video signal and the AFC circuit is disturbed by the noise. As a result, the operation is very unstable. Further, if the response speed of the AFC circuit is increased, the AFC circuit goes beyond the synchronization pull-in range if the time base error is large and the time base error cannot be compensated.
On the other hand, a new standard of television signal system is under discussion to increase a realistic feeling and a resolution power to compare with those of a current television signal system. This system which aims at high quality of image and sound is called a high-definition television system or so-called MUSE (Multiple Sub-Nyquist Encoding). In the high-definition television system, horizontal and vertical resolution powers are approximately twice as high as those of the currently used television system. Accordingly, a frequency band required for transmission is approximately four times as wide as the currently used television signal band.
When the magnetic recording and reproducing apparatus for the currently used television signal is used to record and reproduce the high-definition television signal, only one quarter of the frequency band of the high-definition television signal can be recorded and reproduced.
In the helical scan type magnetic recording and reproducing head, the high-definition television signal can be recorded if the rotation speed of the head cylinder is increased four times.
FIG. 7 shows reproduced signals of a video signal by a two-head helical scan type magnetic recording and reproducing apparatus. FIG. 7a shows waveforms of recorded and reproduced video signals for the currently used television signal, FIG. 7c shows waveforms of recorded and reproduced video signals for the high-definition television signal with the rotation speed of the head cylinder increased four times, FIG. 7b shows a head switching signal used when the current television signal is recorded and reproduced at the normal rotation speed of the head cylinder, and FIG. 7d shows a head switching signal used when the high-definition television signal is recorded and reproduced with the head cylinder rotated at the speed of four times.
In the helical scan type magnetic recording and reproducing apparatus, if the recorded track length expands or shrinks due to change in tape tension, or the tape itself expands or shrinks, or the signal is reproduced by a different apparatus than the apparatus used to record the signal and there is a slight difference between the head mount positions or the cylinder diameters of those two apparatus, there is a time difference between the signal reproduced by a first head and the signal reproduced by a second head at a head switching area.
FIG. 8 shows an enlarged video of the reproduced signals shown in FIGS. 3a or 3c in the vicinity of the signal switching area. FIG. 8e shows the signal reproduced by the first head and FIG. 8f shows the signal reproduced by the second head. The actual recorded and reproduced signals are frequency-modulated but the video signals are shown in FIG. 8 to illustrate the time discontinuity. FIG. 8g shows a head switching signal and FIG. 8h shows a series of video signal after switching from the signal reproduced by the first head to the signal reproduced by the second head.
As described above, there occurs a time difference between the signal reproduced by the first head and the signal reproduced by the second head due to the expansion or shrinkage of the track length. As a result, when the reproduced signal is switched, the video signal at the switching point is discontinuous as shown by h.sub.1 in FIG. 8h and the horizontal synchronization signal is phase-discontinuous as shown by HS.
FIG. 9 shows a reproduced image on a television screen when the signal is recorded and reproduced by the head cylinder rotated at the speed of four times. Since the phase of the horizontal synchronization signal is discontinuous at head switching points T.sub.1, T.sub.2 and T.sub.3 the vertical image on the screen is discontinuous and skew distortions SK appear in the pull-in period due to a time constant of a horizontal synchronization AFC circuit of a television receiver.
In the case of the normal rotation speed shown in FIGS. 7a and 7b, only one skew distortion appears in one vertical scan period and it appears in a vertical blanking period and a raster overscan area. Accordingly, the skew distortion is not essentially observed on the television screen.
In the case of the four times rotation speed shown in FIGS. 7c and 7d, four skew distortions appear in one vertical scan period and at least three skew distortions appear on the screen as shown in FIG. 9. Accordingly, the image quality of the television screen is highly degraded.